I was looking around today to see what people have come up with for solid starter themes based on Underscores, but with some more opinions built in. Surprisingly, I didn't find as many as I hoped, but the one from Design.org did stand out. I've mentioned it once before, but if you haven't seen it, check it out. I quite like the decisions they've made.
I also noticed that default Underscores doesn't include a Grunt or Gulp setup to help compile Sass, if Sass is enabled. I found that interesting, and also thought it was curious that there were basically no docs on working with Sass and Underscores. So many of us point to that theme as a great forkable starter theme, but if I were a beginner there's a pretty good chance I'd get lost. Just food for thought.